The health, nutrient requirements, performance, reproduction and, in extreme cases, survival of fish are all dependent on the temperature of the water. Temperature requirements vary among species as well as between estuarine and freshwater
fish (Tomasso 1993). Very gradual equilibration of water temperature is crucial when transferring, shipping, breeding, and acclimating fish, as well as when adjusting water temperature. (Extreme caution must be taken with all electrical connections. Tanks should be well insulated and grounded and appropriate ground fault circuit breakers should be used.) An optimal temperature variation is about l°C/hour (Tomasso 1993), which is not considered stressful. Temperature variation also affects saturation of gases including oxygen (for example, there is less dissolved oxygen at higher temperatures). Gas content is inversely proportional to the water temperature. Sudden, large increases in temperature pose a hazard to the fish, particularly in a closed container aquarium.